Housing
What size is your tank? 1.5 gallons
What temperature is your tank? 78 F
Does your tank have a filter? No
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? flakes
How often do you feed your betta fish? once or twice a day

Maintenance
How often do you perform a water change? Every 3-5 days
What percentage of the water do you change when you perform a water change? Usually 100%
What type of additives do you add to the water when you perform a water change? Dechlorinater

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Symptoms and Treatment
How has your betta fish's appearance changed? No change
How has your betta fish's behavior changed? Less active, keeps his mouth open when he's resting
When did you start noticing the symptoms? A few days ago
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? He's had problems with bloating a couple times, nothing else that I can remember
How old is your fish (approximately)? I don't know, but I've had him almost exactly a year and he was already full-grown then

About a week ago, Excalibur spent a whole night hiding inside a decoration in his tank. When I fed him and he didn't come out the next morning, I was worried he'd gotten stuck and gently removed the decoration. At the time he seemed dazed for a few minutes, but then he was acting normally and eating. Earlier this week, he suddenly became less active. Sometimes he'll act completely normal, but other times he'll rest for hours at a time and doesn't even notice right away when I feed him. He also sometimes rests with his mouth wide open, which isn't something he used to do. My mother is wondering if what happened last week could have damaged his brain. Is that a possibility? I'm also wondering if these are signs of old age, but the changes happened pretty suddenly. Another possibility I thought of is that it's something wrong with the water, because it's been developing film on top, but I'm pretty sure it's just protein film, and it didn't seem to be bothering him earlier. Does anyone know what the problem is ? I'm really worried about him.

I think he was trapped inside his decoration overnight. So he probably went hours without getting any air.

I've been skimming the film off with a paper towel every day. I have filters for the tanks that I've never used, so I'm trying one out in another tank right now, and I might be putting them in the other two tanks later.

There is no visible sign of damage. Not even stress stripes or a clamped tail. I think it was about 8 days ago that he got caught in his decoration, and then I cleaned his tank on Tuesday, so 4 days ago.

Today he's worse, he is still reacting to me sometimes and is eating, but other times he seems to be having trouble breathing. My family is going to be away from home for a little over 24 hours to go to a dance competition this weekend and I'm really worried about him. It's too late to change plans and stay home but I just have a feeling he'll be gone when we get back. Is there anything I can do for him? Would it help or hurt to do a water change right before we leave? I could probably ask my brother to stop by and feed him, would that help? I thought about moving him to a smaller tank, but he hates being in there when I'm doing water changes, so I feel that would just stress him out. I just feel so awful for him, I need to do something.

If he makes it through the weekend, I'll take a water sample to the pet store on Monday. We have both a Petco and a Petsmart, which one would be better? I'll see what kinds of testing strips they have while I'm there, but last time I checked they didn't have anything under $30.

There is no visible sign of damage. Not even stress stripes or a clamped tail. I think it was about 8 days ago that he got caught in his decoration, and then I cleaned his tank on Tuesday, so 4 days ago.

Today he's worse, he is still reacting to me sometimes and is eating, but other times he seems to be having trouble breathing. My family is going to be away from home for a little over 24 hours to go to a dance competition this weekend and I'm really worried about him. It's too late to change plans and stay home but I just have a feeling he'll be gone when we get back. Is there anything I can do for him? Would it help or hurt to do a water change right before we leave? I could probably ask my brother to stop by and feed him, would that help? I thought about moving him to a smaller tank, but he hates being in there when I'm doing water changes, so I feel that would just stress him out. I just feel so awful for him, I need to do something.

If he makes it through the weekend, I'll take a water sample to the pet store on Monday. We have both a Petco and a Petsmart, which one would be better? I'll see what kinds of testing strips they have while I'm there, but last time I checked they didn't have anything under $30.

The test stripes are a tad expensive. That is another reason why most users suggest the liquid test. It's usually priced around $30. Lasts longer/it is more accurate.
Both stores carry the same test products. Petsmart will have them on sale more often than Petco. And both stores use the same thing(the test strips) to test water samples.

ETA: to the bold: My Spike went through ammonia poisoning maybe 2 weeks ago. He did not lose his color. He did not show stress stripes and the ONLY sign he gave me was laying around just about 24/7. Some fish will show more signs than others. And some fish will show limited signs.

Personally, I'd do a 100% water change/SLOWLY acclimate him into the new water. Float him in his cup(or a sandwich baggie) with 1/4 of his original water. Add a teaspoon of the new water every 15 -20 mins.